1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus having photoelectric conversion elements arrayed in a matrix and being capable of obtaining a high-quality image.
2. Related Background Art
FIG. 1 is a diagram for explaining a conventional photoelectric conversion apparatus. Referring to FIG. 1, photoelectric conversion elements (e.g., photodiodes) 1 store charges in accordance with the amounts of incident light and form a two-dimensional array (4×4 elements in FIG. 1). One terminal of the photoelectric conversion element 1 is connected to the gate of a source follower input MOS 2. The source of the source follower input MOS 2 is connected to the drain of a vertical selection switch MOS 3. The drain of the source follower input MOS 2 is connected to a power supply terminal 5 through a power supply line 4. The source of the vertical selection switch MOS 3 is connected to a load power supply 7 through a vertical output line 6. The source follower input MOS 2, the vertical selection switch MOS 3, and load power supply 7 form a source follower circuit. The photoelectric conversion element 1, the source follower input MOS 2, the vertical selection switch MOS 3, and load power supply 7 form a pixel.
A signal voltage of the photoelectric conversion element 1 is induced at the gate of the source follower input MOS 2 in accordance with the charge accumulated in the photoelectric conversion element of each pixel. This signal voltage is current-amplified and read by the source follower circuit.
The gate of the vertical selection switch MOS 3 is connected to a vertical scanning circuit 9 via a vertical gate line 8. An output signal from the source follower circuit is externally output via the vertical output line 6, a horizontal transfer MOS switch 10, a horizontal output line 11, and an output amplifier 12. The gate of each horizontal transfer MOS switch 10 is connected to a horizontal scanning circuit 13. With this arrangement, the signal voltages of the respective photoelectric conversion elements sequentially turn on the vertical selection switch MOSs 3 by the pulse voltages on the vertical gate lines 8 connected to the vertical scanning circuit 9. The signal voltages are read onto the corresponding vertical lines. The horizontal transfer MOS switches 10 are sequentially turned on by a shift register signal of the horizontal scanning circuit 13. The signal voltages of the respective photoelectric conversion elements are output from the output amplifier 12 as time-serial signals in units of pixels.
In the prior art described above, since finite resistances are distributed in the vertical output lines 6, shading in the vertical direction occurs in the signals due to potential drops across the resistances. For descriptive convenience, one pixel and its peripheral portion are illustrated in FIG. 2. Referring to FIG. 2, a resistance 201 is distributed on the vertical output line 6. Let M rows of pixels be present, and r1 be the resistance value of the vertical output line per row. Then, the total resistance between the pixels on the Kth row and the horizontal transfer MOS switch 10 is defined as:r1×K(1≦K≦M)  (1)
Let Ia, Rm, Vth0, and Vsig0 be the current flowing through the load power supply 7, the series resistance of the vertical selection switch MOSs 3, the threshold voltage of the source follower input MOS 2, and the signal voltage on the gate of the source follower input MOS 2, respectively. Then, a signal Vsig1 current-amplified and read by the source follower circuit is defined as:Vsig1=Vsig0−Vth0−Ia×Rm−Ia×r1×K(1≦K>M)   (2)That is, even if the identical signal voltages Vsig0 is induced at the pixels, the voltages Vsig1 read in units of rows have differences due to voltage drops by the resistances r1 of the vertical output lines 6, thus causing vertical shading. The image quality is greatly deteriorated.
In recent years, the number of pixels increases and the size decreases in the development of photoelectric conversion apparatus. The wirings used in the photoelectric conversion apparatus tend to be thin and long. Voltage drop by the resistance r1 of the vertical output line 6 poses a serious problem.
Another problem is posed by different dynamic ranges of the source follower circuit in units of rows because a finite resistance is distributed on the power supply line 4. This problem will be described with reference to FIG. 2. A resistance 202 in FIG. 8 is distributed on the power supply line 4. Let M rows of pixels be present, and r2 be the resistance value of the power supply line per row. Then, the total resistance between the pixels on the Kth row and the power supply terminal 5 is:r2×K(1≦K≦M)  (3)
Letting Vd be the voltage of the power supply terminal 5, the source follower input MOS 2 must operate as a pentode in order to operate the source follower circuit as a linear amplifier. A condition for this is given by:Vd−Ia×r2×K>Vsig0−Vth0(1≦K≦M)  (4)The above condition can be rewritten as: Vsig0<Vd+Vth0−Ia×r2×K(1≦K≦M)  (5)
The signal voltage values not satisfying the above condition are different depending on the rows. That is, the signals have different dynamic ranges.
This results in saturation voltage shading or output shading on the small-light-amount characteristic side due to a combination with the polarities of the photodiode 1, thereby greatly degrading the image quality.